JP2010034246A - Semiconductor laser element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nitride semiconductor laser element which is low in operating voltage and stably has long-period reliability. <P>SOLUTION: The semiconductor laser element has a lower clad layer 2 of a first conductivity type, an active layer 5, and an upper clad layer 8 of a second conductivity type formed in order on a substrate 1, and a first electrode 11 formed thereupon, wherein the upper clad layer is in a ridge shape such that a striped ridge portion S1 continues from a first surface S2 and projects, and the lower clad layer, active layer, and upper clad layer are formed of nitride semiconductor layers. The first electrode is provided in contact with an upper surface of the ridge portion and at least a partial region of a sidewall of the ridge portion, the first surface is covered with an insulating film, and the first electrode is not in contact. Consequently, electric charge generated owing to spontaneous polarization and piezoelectric polarization can be canceled, so the voltage can be lowered. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a laser device using a semiconductor, and more particularly to a nitride semiconductor laser device.

In recent years, research and development of GaN-based nitride semiconductor light emitting devices that emit light from the ultraviolet region to blue have been actively conducted. In particular, research and development of semiconductor laser elements using a nitride semiconductor as an optical disk with higher density and a laser display are actively conducted. This nitride semiconductor laser device is disclosed in, for example, the following paper (Non-Patent Document 1) by Shuji Nakamura et al.
APPLIED PHYSICS LETTERS VOLUME 72, NUMBER 16 (1998) 2014-2016 `` Continuous-wave operation of InGaN / GaN / AlGaN-based laser diodes grown on GaN substrates ''

The paper (Non-Patent Document 1) describes a laser cross-sectional structure as shown in FIG. 2 (a diagram showing a cross-sectional structure of a conventional nitride semiconductor laser device). 2, 51 is an n-type cladding layer having an n-AlGaN / GaN superlattice structure, 52 is an n-GaN layer, 53 is an active layer having a multiple quantum well (MQW) structure made of InGaN, and 54 is a p-AlGaN electron stopper. Layer, 55 is a p-GaN layer, 56 is a p-type cladding layer having a p-AlGaN / GaN superlattice structure, 57 is a p-GaN layer, 58 is a p-electrode (anode electrode), 59 is silicon oxide (SiO ₂ ) Is a layer.

A part of the p-type cladding layer 56 is processed by dry etching so that the striped ridge portion S1 is processed into a ridge shape protruding continuously from the first surface S2, and the ridge sidewall of the p-type cladding layer 56 (the sidewall of the ridge portion S1) And an insulating film 59 made of, for example, silicon oxide (SiO ₂ ) is in contact with the first surface S2 at the bottom of the ridge. Only the upper portion of the ridge (the upper surface of the ridge portion S1) has no SiO ₂ film (insulating film 59), and the p-electrode 58 is in contact therewith. Below the n-type cladding layer 51 are a GaN semiconductor substrate and an n-electrode (cathode side electrode), which are omitted here.

  Such a nitride semiconductor laser device has a problem of high operating voltage. This tendency is particularly noticeable when the p-type cladding layer 56 is made of bulk AlGaN. This is due to the asymmetry of the crystal, and surface charges are generated up and down due to the spontaneous polarization and piezoelectric polarization of AlGaN. In FIG. 2, a negative charge is generated at the upper end of the p-type cladding layer 56 and a positive charge is generated at the lower end. Since the p-type cladding layer 56 is thick, a voltage in the direction opposite to the forward direction of the semiconductor laser is applied to the p-type cladding layer 56. In order to pass a current in the forward direction, the voltage generated by spontaneous polarization is canceled. As much voltage as possible is required. Further, it is known that a nitride semiconductor p-type conductive layer has a high resistivity due to extremely low hole mobility. Furthermore, the contact resistance between the p-type nitride semiconductor and the p-electrode is large due to the fact that the nitride semiconductor is a wide gap semiconductor. Therefore, as shown in FIG. 2, the resistance increases in the ridge shape in which the p-type nitride semiconductor is narrowed in a rectangular shape.

  FIG. 3 shows an example of current-voltage characteristics of a nitride semiconductor laser element actually manufactured. The device structure is almost the same as that shown in FIG. 2, but the p-type cladding layer uses bulk p-type AlGaN, and the substrate is an n-type GaN semiconductor substrate. An n-electrode (cathode side electrode) is formed under the substrate. ). At this time, the threshold current of the nitride semiconductor laser element was 42 mA, and the oscillation wavelength was 415 nm. The applied voltage calculated from the oscillation wavelength should be slightly larger than about 3V (about 0.2V to 0.5V). However, as can be seen from FIG. 3, a very large voltage of 6 to 8 V was shown. This characteristic was slightly dependent on the mesa width of the ridge shape and showed a high voltage characteristic. Furthermore, when this nitride semiconductor laser device was subjected to an energization test for 8 hours, the operating voltage increased by 1.5 to 3 V, and the threshold current also increased by 15 to 30%. That is, it was found that the laser characteristics were greatly deteriorated and reliability was not obtained even in a short time.

  On the other hand, the dotted line in the figure is the current-voltage characteristic of conducting the same wafer without passing through the p-type cladding layer. The voltage is very small, 4-5V. The reason why the contact area with the electrode is about 1V or less than that of a normal laser is about 1V larger than the approximate calculation value because the resistance component resulting from it is large.

  From these comparisons, it can be seen that the voltage component related to the p-type cladding layer is large. As described above, this is due to spontaneous polarization and piezoelectric polarization, high resistance of the nitride p-type semiconductor, and high contact resistance between the nitride semiconductor and the metal. Since the mesa width dependence is extremely small, it is assumed that the influence of spontaneous polarization and piezo polarization is large. This high voltage tendency is improved when the p-type cladding layer 56 has a superlattice structure. However, the effect cannot be completely eliminated. Further, since the superlattice structure has a complicated structure in which several hundred semiconductor layers having different compositions of several nm are grown, the manufacturing process is complicated, leading to an increase in manufacturing cost.

  Examples of the ridge type semiconductor laser element include InGaAlAs and InGaAsP type ridge type semiconductor laser elements. In these ridge-type semiconductor laser elements, Au that enters the ridge portion from the electrode (Ti / Pt / Au) formed so as to cover the ridge portion through the sidewall of the ridge portion by covering the sidewall of the ridge portion with an insulating film. Suppression of the diffusion.

  The problem to be solved by the present invention is to provide a nitride semiconductor laser device that has a low operating voltage and is stable and can provide long-term reliability. It is another object of the present invention to provide a nitride semiconductor laser device having a low manufacturing cost and a low operating voltage.

  Of the inventions disclosed in this application, the outline of typical ones will be briefly described as follows.

The present inventors have intensively studied to solve the above problems. The outline is as follows.
The inventors have conducted various studies and found that the operating voltage can be reduced by bringing the p-type nitride semiconductor layer on the side wall as well as the upper portion of the ridge portion into contact with the electrode. This is presumably because charges generated by spontaneous polarization and piezo polarization of the p-type cladding layer can be canceled by the electrode on the side wall. Furthermore, since a current path from the lateral direction is added to the p-type nitride semiconductor having a high resistivity by applying this structure, the substantial resistance in the p-type cladding layer is reduced.

  Note that the side wall (ridge side wall) of the ridge formed by dry etching is moderately rough, and is effective for obtaining electrical contact even if the carrier concentration is somewhat low. Further, in this structure, since the electrode area increases, the contact resistance between the originally large nitride semiconductor and the p-electrode can be reduced.

  Specifically, in the present invention, the above-described problems are the first conductivity type lower cladding layer formed on the substrate, the active layer formed on the lower cladding layer, and the active layer formed on the active layer, A second conductivity type upper cladding layer having a conductivity type opposite to the first conductivity type; and a first electrode formed on the upper cladding layer, wherein the upper cladding layer has a striped ridge portion. Each of the lower cladding layer, the active layer, and the upper cladding layer is a nitride semiconductor laser element made of a nitride semiconductor layer, and has a ridge shape protruding continuously from the first surface. The first electrode is provided in contact with the upper surface of the ridge portion and at least a partial region of the side wall of the ridge portion, the first surface is covered with an insulating film, and the first surface is covered with the first surface. Nitride half, characterized in that the electrodes are not in contact It is achieved by the body laser element.

  The above and other objects and novel features of the present invention will become apparent from the description of this specification and the accompanying drawings.

The effects obtained by the representative ones of the inventions disclosed in the present application will be briefly described as follows.
According to the present invention, it is possible to provide a nitride semiconductor laser element that has a small operating voltage and can stably obtain long-term reliability.
In addition, according to the present invention, it is possible to provide a nitride semiconductor laser device having a low manufacturing cost and a low operating voltage.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In all the drawings for explaining the embodiments of the invention, those having the same function are given the same reference numerals, and their repeated explanation is omitted.

1 is a cross-sectional view showing a cross-sectional structure of a nitride semiconductor laser device that is Embodiment 1 of the present invention.
The nitride semiconductor laser device of Example 1 has a laser structure in which crystals are grown on an n-type GaN semiconductor substrate 1 by MOCVD (Metal-Organic Chemical Vapor Deposition) method. From the viewpoint of ease of crystal growth, it is preferable to use a substrate in which the Ga polar face of the GaN substrate 1 faces the upper surface direction. From the bottom, an n-type cladding layer 2 made of a nitride semiconductor of n-AlGaN 2, an n-GaN layer 3, an InGaN guide layer 4, an active layer 5 having a multiple quantum well (MQW) structure made of InGaN 5, an InGaN guide layer 6, p- The AlGaN electron stopper layer 7, the p-type cladding layer 8 made of p-AlGaN nitride semiconductor, and the p-type contact layer 9 made of p ⁺ -GaN nitride semiconductor are crystal-grown, and the p-type contact layer 9 and the p-type cladding are formed. A part of the layer 8 has a ridge shape. That is, the p-type cladding layer 8 has a ridge shape in which the striped ridge portion S1 projects continuously from the first surface S2 of the bottom side of the ridge, and the ridge portion S1 includes the p-type cladding layer 8 and the p-type cladding layer 8. The p-type contact layer 9 is provided on the cladding layer 8.

The p-type cladding layer 8 is doped with Mg, and the impurity concentration is preferably in the range of 1 to 3 × 10 ¹⁹ cm ⁻³ . The height of the ridge portion S1 is 500 nm, the width of the ridge portion S1 is 1.6 μm, and has a rectangular or trapezoidal shape. The height of the ridge portion S1 is preferably 400 to 800 nm. The p-type contact layer 9 may be made of AlGaN.

  An insulating film 10 made of SiNx and an adhesive layer 21 made of amorphous Si are stacked on the first surface S2 at the bottom of the ridge. The thickness of the insulating film 10 is 150 nm, and the thickness of the adhesive layer 21 is 10 nm. Nos. 10 and 21 use the film quality change of the film forming apparatus, the film quality is poor on the side wall (ridge side wall) of the ridge portion S1, and a good quality film is attached to the first surface S2 of the ridge side bottom. Can be easily removed, and a structure that covers the entire ridge side bottom as shown in FIG.

  The height at which the insulating film 10 covers the ridge side wall can be 150 nm, which is the same height as the film thickness, or can be higher or lower depending on the film forming conditions and the etching method and conditions. The lower limit is the intersection of the ridge side wall and the ridge side bottom (the portion where the side wall of the ridge S1 and the first surface S2 of the ridge side bottom are continuous). A desirable range of this height will be described later.

  Reference numeral 11 denotes a p-electrode (anode side electrode) which covers the side wall from the ridge-shaped upper surface of the p-type cladding layer 8 and is in electrical contact. For example, Pd is used as the p-electrode 11. Since the insulating film 10 and the adhesive layer 21 reach the side wall of the ridge, the p-electrode 11 is not in contact with the first surface S2 at the side of the ridge, so that current flows from the ridge portion S1 only to the active layer 5. Can be injected. That is, the p-electrode 11 is provided in contact with the upper surface of the ridge portion S1 and at least a partial region of the side wall of the ridge portion S1, and the first surface S2 of the ridge side bottom portion is covered with the insulating film 10, and the ridge side bottom portion The p-electrode 11 is not in contact with the first surface S2.

Furthermore, a pad electrode 12 made of a multilayer metal of Ti / Pt / Au is provided so as to cover and electrically contact the p electrode 11. An n electrode (cathode side electrode) 13 in electrical contact is provided under the n-GaN substrate 1. The p electrode 11 is preferably made of Au, Pd, Pt, Co, or Ni as a metal that obtains good ohmic characteristics with the p-type nitride semiconductor. More preferably, Pd is used. The film thickness is preferably 10 to 500 nm. Of these, Au, Pt, and Pd, which are noble metals, may have poor adhesion to an insulator, and therefore, in Example 1, a highly adhesive film and an adhesive layer 21 are stacked on the insulating film 11. As the adhesive layer 21, a metal oxide such as W, Mo, Hf, Al, or Ti, amorphous Si, or poly-Si is used. Preferably amorphous Si is used. As the insulating film 11, Si oxide or nitride, Al oxide or nitride, ZrO ₂ , WO ₃ , TiO ₂ , TiSiN, HfO ₂ or the like is used. Preferably, a nitride of Si (hereinafter abbreviated as SiNx) is used.

Here, FIG. 7 shows a typical example of current-voltage characteristics of the conventional structure similar to FIG. 2 and the structure of the present invention. In FIG. 7, (α) is the current-voltage characteristic of the laser element in the initial state. In the conventional structure, the voltage is about 6 to 7 V with an injection current of several mA to 40 mA, which is large. On the other hand, in the present invention, a very small voltage of 3.5 V to 4.2 V with respect to an injection current of several mA to a little less than 80 mA. Further, (β) is a current-voltage characteristic after energization. (Β) of the conventional structure is a state after operating for 8 hours at a constant current with a current corresponding to a light output of 5 mW in the initial state at 25 ° C., and the voltage increased from 1.5 V to 2 V from the initial state. At this time, the threshold current increased by about 20%, and the slope efficiency decreased by about 1.5%. That is, the laser characteristics have deteriorated.
On the other hand, when an energization test was performed under the same conditions in the structure of the present invention, the current-voltage characteristics hardly changed as shown in FIG. 7, and the threshold current and slope efficiency were less than 2 to 3% measurement error. The rate of change was within the range. As described above, the conventional structure has not only high initial operating voltage but also further increase in operating voltage due to energization and deterioration of laser characteristics, whereas the present invention originally has low operating voltage, and the operating voltage and laser characteristics due to energization. It was possible to realize an excellent laser structure without any deterioration.

  The major difference is whether or not there is an electrode on the ridge sidewall. Next, it was examined how much electrode should be in contact with the ridge side wall. FIG. 6 shows the laser cross-sectional structure above the p-type cladding layer 8, and the height from the lower edge of the ridge shape to the lower edge where the p-electrode 11 is in contact with the ridge side wall as shown in FIG. The height from the first surface S2 of the bottom side of the ridge to the p-electrode 11 in contact with the side wall of the ridge S1 is h. The insulating film 10 is in contact with the ridge side wall below h. FIG. 8 shows a laser structure in which h is changed several times, and an energization test similar to that described above is performed to examine changes in operating voltage. As apparent from FIG. 8, the operating voltage changes rapidly from the point A to the point B, and the height h from the lower end of the ridge shape to the lower end where the p-electrode 11 is in contact with the ridge side wall is It can be seen that 350 nm or less (point A) is preferable. A more preferable range is 300 nm or less (point B). That is, when the height from the first surface S2 of the ridge side bottom portion to the upper surface of the ridge portion S1 is H, the p electrode 11 in contact with the side wall of the ridge portion S1 from the first surface S2 of the ridge side bottom portion. The height h is preferably 0.7H or less, and more preferably 0.6H or less.

  FIG. 4 shows an outline of the laser according to a bird's eye view. Detailed layer structure is omitted. The main region of the ridge stripe is covered with the p electrode 11 and the pad electrode 12. In the vicinity of the end face, electrodes are excluded in order to prevent damage to the ridge shape during cleavage and to prevent end face deterioration due to energization.

  Here, the manufacture of the nitride semiconductor laser device of Example 1 will be described with reference to FIG. FIG. 5 is a diagram showing a manufacturing process of the nitride semiconductor laser device.

First, as shown in FIG. 5A, an n-type cladding layer 2, an n-GaN layer 3, and an InGaN layer made of n-AlGaN are formed on an n-type GaN semiconductor substrate 1 by MOCVD (Metal-Organic Chemical Vapor Deposition). Guide layer 4, active layer 5 having a multiple quantum well (MQW) structure made of InGaN 5, InGaN guide layer 6, p-AlGaN electron stopper layer 7, p-type cladding layer 8 made of p-AlGaN, p made of p ⁺ -GaN The type contact layer 9 is crystal-grown. From the viewpoint of easiness of crystal growth, the GaN semiconductor substrate 1 is preferably grown on the Ga polar face side. The p-type cladding layer 8 is doped with Mg, and the impurity concentration is preferably in the range of 1 to 3 × 10 ¹⁹ cm ⁻³ , and the Mg impurity concentration is preferably higher than that of the p-type contact layer 9.

Next, a dielectric film 30 is formed on the p-type contact layer 9 by a CVD apparatus or the like. The dielectric film 30 is, for example, SiO ₂ . Thereafter, a photoresist is applied, a stripe-shaped photoresist 31 is formed by a photomask and an exposure apparatus, and further, the dielectric film 30 is dry-etched or wet-etched using the stripe-shaped photoresist 31 as a mask, As shown in FIG. 5B, the stripe shape is transferred to the dielectric film 30 by etching.

  Next, the photoresist 31 is removed, and the p-type contact layer 9 and a part of the p-type cladding layer 8 are etched by chlorine-based dry etching to form a striped ridge portion S1 as shown in FIG. Forms a ridge shape protruding continuously from the first surface S2 of the ridge side bottom.

  Next, as shown in FIG. 5D, the insulating film 10 and the adhesive layer 21 are formed by a film forming apparatus such as a sputtering apparatus. At this time, since the film quality is poor on the ridge side wall (side wall of the ridge portion S1) and a good quality film is attached to the first surface S2 of the ridge side bottom portion, only the ridge side wall can be easily removed by wet etching. At this time, the dielectric film 30 can also be removed by wet etching, and the insulating film 10 and the adhesive layer 21 adhering thereto can also be removed.

  Next, as shown in FIG. 5E, a p-electrode 11 provided in contact with the upper surface of the ridge portion S1 and at least a partial region of the side wall is formed. The p electrode 11 is formed by a lift-off method. Specifically, a resist film having an opening wider than the width of the ridge S1 is formed by a photolithography technique at a position corresponding to the ridge S1, and then the resist film including the inside of the opening is formed on the resist film. A conductive film is formed, and then the resist film is removed and the conductive film on the resist film is selectively removed (lifted off).

  Next, as shown in FIG. 5F, a pad electrode 12 that covers the p-electrode 11 is formed. The pad electrode 12 is also formed by the lift-off method, like the p electrode 11.

  The semiconductor laser device according to Example 1 has good characteristics such as an oscillation wavelength of 443 nm, a threshold current of 33 mA, a slope efficiency of 1.43 W / A, an operating voltage of 3.89 V at a threshold current at room temperature, In a 5 mW APC (Auto Power Control) test, good reliability characteristics were obtained with an increase rate of operating current of 8% or less over 1500 hours. At this time, a very stable operation with a fluctuation of operating voltage of several percent or less could be obtained.

As for the insulating film 10, the SiNx film has a voltage reduction effect of about 0.5 V or more than the SiO ₂ film even at the same insulating film height. Although this cause is not certain, it is thought to be due to the state of the semiconductor-insulating film interface. From this point of view, the SiNx film can be said to be a preferable form. In this embodiment, a bulk p-AlGaN layer is used as the p-cladding layer, but the effect can be obtained by applying a p-AlGaN / GaN superlattice structure. Even in the superlattice structure, the spontaneous polarization and piezo polarization cannot be completely eliminated, and the resistance value of the p-cladding layer is reduced by adding a current path from the lateral direction and the contact resistance is reduced by increasing the contact area of the p-electrode. Because there is.

  In the superlattice structure, the effect of this structure does not change even if either the AlGaN layer or the GaN layer is p-doped. Further, with respect to the width of the ridge portion, it was possible to confirm the voltage reduction effect by this structure up to 12 μm. Furthermore, in this embodiment, a part of the upper part of the p-type cladding layer 8 made of p-AlGaN has a ridge shape, but the entire p-type cladding layer 8 may have a ridge shape. In this case, a p-type semiconductor layer such as a p-GaN layer may be inserted between the p-type cladding layer 8 and the p-AlGaN electron stopper layer 7.

Here, the forms of the p electrode 11, the p-type cladding layer 8, the insulating film 10, and the like will be described with reference to FIGS. 9 and 10. FIG. 9 is a diagram illustrating first to fourth modifications of the first embodiment, and FIG. 10 is a diagram illustrating fifth to seventh modifications of the first embodiment.
It is important that the p electrode 11 is in electrical contact with the ridge side wall. Therefore, as in the first modification of FIG. 9A, the p-electrode 11 is not on the insulating film 10 (the p-electrode 11 is not formed from the ridge side wall to the insulating film 10), and the ridge top surface Alternatively, the shape may be only on the ridge side wall.

  9B, the p-electrode 11 is formed on the ridge upper part and the ridge side wall before the insulating film 10, and then the insulating film 10 is formed. Alternatively, a structure sandwiched between the insulating film 10 and the p-type cladding layer 8 may be used.

  Further, as in the third modified example of FIG. 9C, the p electrode 11 is only on the ridge or only on the ridge and a part of the ridge side wall, and the pad electrode 12 is in contact with the ridge side wall below the p electrode 11. It may be a form. In this case, the ridge side wall and the pad electrode (for example, Ti / Pt / Au) 12 are in Schottky connection, but the effect of canceling spontaneous polarization and piezoelectric polarization of the p-type cladding layer 8 is maintained. Further, the effect of substantially reducing the resistance value of the p-type cladding layer 8 due to the addition of a current path from the lateral direction is also maintained.

  Furthermore, a structure that does not use the adhesive layer 21 shown in FIG. 1 can be applied as in the fourth modified example of FIG. In this case, if the p electrode 11 is selected to have good adhesiveness with the insulating film 10, or if the pad electrode 12 is selected to have good adhesiveness with the insulating film 10, the p electrode 11 will be enclosed, and the p electrode 11 will be completed. As a structure, stability can be maintained.

  As a form of the p-type cladding layer 8, in addition to the bulk p-AlGaN cladding layer and the p-AlGaN / GaN superlattice structure cladding layer described above, as in the fifth modification of FIG. There is a form of a p-type cladding layer having a composite structure of a p-type cladding layer 22b having an AlGaN / GaN superlattice structure and a p-type cladding layer 22a made of p-AlGaN. This is because it is effective to use a superlattice cladding layer so that the p-type dopant does not diffuse into the guide layer and the active layer at the bottom of this figure when the reliability factor is diffusion of the p-type dopant (Mg, etc.). There is. Moreover, it can be said that a complex structure simplifies the growth process of a complicated superlattice structure and is a structure that is easier to manufacture.

  The sixth modification of FIG. 10B has a structure in which a p-type contact layer 23 made of an AlGaN layer or a GaN layer with a higher concentration of p dopant is regrown after forming the ridge shape and forming the insulating film 10. is there. Thereafter, when the p-electrode 11 is formed, good ohmic contact is obtained not only on the ridge upper surface but also on the ridge sidewall, and the contact resistance can be lowered. The effect of canceling the spontaneous polarization or piezo polarization of the p-type cladding layer 8 and the effect of reducing the substantial resistance value of the p-type cladding layer 8 by adding a current path from the lateral direction can be equal or better.

  Further, as in the seventh modification of FIG. 10C, a structure in which the lower end of the p-type cladding layer 8 has a bottom, that is, the side surface of the ridge portion S1 and the first surface S2 of the ridge side bottom portion, If the cross-sectional shape of the continuous portion is a gently curved shape, the contact between the p-type cladding layer 8 (semiconductor) and the insulating film 10 at the ridge side wall and the bottom of the ridge, and the p electrode 11 and the insulating film In this region, the p-type electrode 11 can be prevented from coming into contact with the first surface S2 at the bottom of the ridge due to poor formation of the insulating film 10 in that region. Optically, the refractive index does not change sharply at the lower end of the ridge, and the current spreads straight from the lower end of the ridge, so that the far-field image of the laser light is not disturbed and a unimodal Gaussian function shape is obtained.

  In this embodiment, a pure blue laser was described as an example. Further, a nitride semiconductor laser element using green InGaN with increased In as an active layer, or a green nitride semiconductor using InN in a quantum well. It goes without saying that the present invention can also be applied to laser elements.

  In the present embodiment, the nitride semiconductor formed on the n-GaN semiconductor substrate 1 has been described. However, if a similar structure can be formed on a substrate on which a nitride semiconductor layer can be grown, for example, a sapphire substrate, a ZnO substrate, or the like. Needless to say, an effect can be obtained.

  In the case of using a non-conductive substrate, it goes without saying that the nitride semiconductor is etched from a remote substrate while avoiding the ridge structure to expose the n-type cladding layer and take the n-electrode pad.

  In the present embodiment, the insulating film on the bottom of the ridge may be covered as long as the element functions. That is, it is only necessary to cover the region including the pad so as to be about one size larger than the electrode, and the region without the electrode in the peripheral portion of the semiconductor chip may have no insulating film.

FIG. 11 is a cross-sectional view showing a cross-sectional structure of a nitride semiconductor laser element that is Embodiment 2 of the present invention.
A second embodiment of the present invention will be described with reference to FIG. The layer configuration of the nitride semiconductor laser device of FIG. 11 is the same as that of the first embodiment. However, while the ridge shape of the first embodiment is a normal rectangle or trapezoid, the second embodiment has a structure in which a narrow ridge shape is put on a wide ridge shape. That is, the ridge portion S1 of the second embodiment has a first portion S1a projecting continuously from the first surface S2 of the ridge side bottom portion, and a projecting portion projecting from the first portion S1a, and the first portion S1a. It has a two-stage structure having a second portion S1b having a width narrower than the first width. The width of the wide lower ridge shape (first portion S1a) is 2 to 3 μm, and the width of the upper ridge shape (second portion S1b) is 1.2 to 1.8 μm. The height of the lower ridge (the height from the first surface S2 to the upper surface of the first portion S1a) is 180 nm, and the height of the upper ridge shape (the second surface from the upper surface of the first portion S1a) The height to the upper surface of the portion S1b) is 240 nm.

  The p-electrode 11 is in electrical contact with not only the upper ridge sidewall but also the lower ridge upper surface and sidewall. That is, the p-electrode 11 is formed on the upper surface and the side wall of the second portion S1b of the ridge portion S1, the upper surface of the first portion S1a of the ridge portion S1, and at least a partial region of the first portion S1a of the ridge portion S1. It is provided in contact. Thereby, a voltage can be reduced significantly. Furthermore, since the width of the upper ridge shape is narrow, the far-field image of the laser beam is not disturbed, and a unimodal Gaussian function far-field image can be obtained. The nitride semiconductor laser device according to this example has good characteristics such as an oscillation wavelength of 405 nm at room temperature, a threshold current of 43 mA, a slope efficiency of 1.51 W / A, an operating voltage of 3.53 V at the threshold current, and 70 ° C. In the 25 mW APC (Auto Power Control) test, good reliability characteristics were obtained with an operating current increase rate of 4% or less over 3000 hours. At this time, a very stable operation with a fluctuation of operating voltage of several percent or less could be obtained.

  Similar to Example 1 described above, the p-type cladding layer 8 can obtain the same effect even in a superlattice structure, and the various forms shown in FIGS. It goes without saying that it is obtained.

12 is a cross-sectional view showing a cross-sectional structure of a nitride semiconductor laser device according to Example 3 of the present invention, FIG. 13 is a view showing a first modification of Example 3, and FIG. FIG. 10 is a diagram illustrating a second modification of the third embodiment.
A third embodiment of the present invention will be described with reference to FIG. The layer configuration of the nitride semiconductor laser device of FIG. 12 is the same as that of the first embodiment. However, unlike Example 1, a multi-ridge structure having three ridge shapes (ridge portions) is formed. This structure is a phased array structure in which the light from each ridge portion S1 overlaps and interacts when the interval between each ridge shape (ridge portion S1) is as narrow as several μm. Becomes an arrayed shape. In either case, the application of the present invention can realize low voltage and high reliability.

  The layer configuration of the nitride semiconductor laser device of FIG. 12 is the same as that of the first embodiment. However, the ridge side bottoms on both sides of the central ridge shape (ridge portion S1) are characterized in that they are covered with the p electrode 11 without the insulating film 10. Further, the ridge side wall is in contact with the p-electrode 11 in all the ridge portions S1.

  That is, at least two or more (three in this embodiment) ridge portions S1 are provided, and the p-electrode 11 is provided in contact with the upper surface of each ridge portion S1 and at least a partial region of the side wall of each ridge portion S1. It has been. Further, the first surface S2 of the ridge side bottom portion includes a first region S2a sandwiched by the ridge portion S1 and a second region S2b not sandwiched by the ridge portion S1, and the first surface S2 of the first surface S2 The p electrode 11 is in contact with the first region S2a, the second region S2b of the first surface S2 is covered with the insulating film 10, and the p electrode 11 is not in contact with the second region S2b. ing.

  With this structure, voltage increase due to spontaneous polarization or piezo polarization can be suppressed as compared with a simple wide ridge shape, and the substantial resistance value of the p-cladding layer due to the addition of a current path from the lateral direction. The effect of reduction is great. Furthermore, the contact resistance can be reduced.

  The nitride semiconductor laser device according to this example has good characteristics such as an oscillation wavelength of 405 nm, a threshold current of 97 mA, a slope efficiency of 1.92 W / A, an operating voltage of 3.43 V at a threshold current at room temperature, and 50 ° C. In the 350 mW APC (Auto Power Control) test, good reliability characteristics were obtained with an increase in operating current of 4% or less over 5000 hours. At this time, a very stable operation with a fluctuation of operating voltage of several percent or less could be obtained.

On the other hand, the first modification of FIG. 13 is also effective as a form. In FIG. 13, since the first surface S2 of the side bottom of the middle ridge shape (ridge portion S1) is covered with the insulating film 10, it has a structure in which three laser structures of FIG. Yes.
That is, in the modified example of the third embodiment, at least two ridge portions S1 (three in the present modified example) are provided, and the p-electrode 11 includes the upper surface of each ridge portion S1 and the side wall of each ridge portion S1. Is provided in contact with at least a partial region. Further, the first surface S2 of the ridge side bottom portion includes a first region S2a sandwiched by the ridge portion S1 and a second region S2b not sandwiched by the ridge portion S1, and the first surface S2 of the first surface S2 The first and second regions (S2a, S2b) are covered with an insulating film 10, and the first and second regions (S2a, S2b) of the first surface S2 are not in contact with the p-electrode 11. ing.

This has the process advantage that it can be manufactured by the same process steps as in FIG.
Further, when combined with the configuration of the above-described second embodiment, the form according to the second modification of FIG. 14 is also effective. In this case, the substantial resistance can be further reduced.
Similar to the first embodiment, the p-type cladding layer 8 can achieve the same effect even in a superlattice structure, and various forms shown in FIGS. 9 and 10 can be similarly applied as modifications of the first embodiment. Needless to say, an effect can be obtained.

FIG. 15 is a diagram showing a cross-sectional structure of a nitride semiconductor laser device that is Embodiment 4 of the present invention, in which a p-type superlattice cladding layer 40 made of an AlGaN / GaN superlattice layer is applied as a p-type cladding layer. It is. The p-type superlattice cladding layer 40 is obtained by alternately growing about 100 pairs of AlGaN and GaN layers each having a thickness of about 2.5 nm, and the total thickness of the superlattice layer is about 500 nm. The impurity doping of Mg is introduced into either the AlGaN layer or the GaN layer, but the impurity concentration is higher than the bulk and is preferably in the range of 3-8 × 10 ¹⁹ cm ⁻³ . As shown in FIG. 15, the p-type superlattice cladding layer 40 is in contact with the electrode 11 at the ridge side wall. At this time, since the impurity concentration of the doped superlattice layer is higher than that of the bulk, an ohmic junction can be easily obtained, and the contact area of the ohmic junction combined with the region of the p-type contact layer 9 on the upper surface is a p-type formed in bulk. It becomes larger than that of the clad layer, and the resistance can be reduced.

  As mentioned above, the invention made by the present inventor has been specifically described based on the above embodiments. However, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the scope of the invention. Of course.

It is a figure which shows the cross-section of the nitride semiconductor laser element which is Example 1 of this invention. It is a figure which shows the cross-section of the conventional nitride semiconductor laser element. It is a figure which shows the current-voltage characteristic of the conventional nitride semiconductor laser element. 1 is a bird's-eye view showing an external configuration of a nitride semiconductor laser element that is Embodiment 1 of the present invention. It is a figure which shows the manufacturing process of the nitride semiconductor laser element which is Example 1 of this invention. In the nitride semiconductor laser element of this invention, it is a figure which shows the cross-sectional structure above from a p-type clad layer. It is a figure which shows the current-voltage characteristic of the nitride semiconductor laser element of this invention. In the nitride semiconductor laser element of this invention, it is a figure which shows the relationship between the height to an electrode lower end, and operating voltage. It is a figure which shows the 1st-4th modification of Example 1 of this invention. It is a figure which shows the 5th-7th modification of Example 1 of this invention. It is a figure which shows the cross-section of the nitride semiconductor laser element which is Example 2 of this invention. It is a figure which shows the cross-section of the nitride semiconductor laser element which is Example 3 of this invention. It is a figure which shows the 1st modification of Example 3 of this invention. It is a figure which shows the 2nd modification of Example 3 of this invention. It is a figure which shows the cross-section of the nitride semiconductor laser element which is Example 4 of this invention.

Explanation of symbols

1 n-GaN semiconductor substrate 2 n-type cladding layer (n-AlGaN)
3 n-GaN layer 4 InGaN guide layer 5 active layer (InGaN-MQW)
6 InGaN guide layer 7 p-AlGaN electron stopper layer 8 p-type cladding layer 9 p-type contact layer (p ⁺ -(Al) GaN)
10 Insulating film 11 P electrode (anode side electrode)
12 Pad electrode 13 n electrode (cathode side electrode)
21 Adhesive layer 22a p-type cladding layer (p-AlGaN)
22b p-type cladding layer (p-AlGaN / GaN superlattice structure)
23 p-type contact layer 30 dielectric mask 31 photoresist 40 p-type superlattice clad layer 51 n-type clad layer (n-AlGaN / GaN superlattice structure)
52 n-GaN layer 53 Active layer (InGaN MQW)
54 p-AlGaN electron stopper layer 55 p-GaN layer 56 p-type cladding layer (p-AlGaN / GaN superlattice structure)
57 p-GaN layer 58 p electrode (anode side electrode)
59 Silicon oxide (SiO ₂ ) layer

Claims (13)

A first conductivity type lower cladding layer formed on a substrate;
An active layer formed on the lower cladding layer;
An upper cladding layer of a second conductivity type formed on the active layer and having a conductivity type opposite to the first conductivity type;
A first electrode formed on the upper cladding layer;
The upper clad layer has a ridge shape in which a striped ridge portion projects from the first surface,
Each of the lower cladding layer, the active layer, and the upper cladding layer is a nitride semiconductor laser element made of a nitride semiconductor layer,
The first electrode is provided in contact with the upper surface of the ridge portion and at least a partial region of the side wall of the ridge portion,
The nitride semiconductor laser device, wherein the first surface is covered with an insulating film, and the first electrode is not in contact with the first surface. The nitride semiconductor laser device according to claim 1,
The first electrode is formed from an upper surface of the ridge portion to a side wall of the ridge portion, and is separated from the first surface,
2. The nitride semiconductor laser device according to claim 1, wherein the insulating film is in contact with a portion of the side wall of the ridge portion that is not in contact with the first electrode. The nitride semiconductor laser device according to claim 2,
When the height from the first surface to the upper surface of the ridge portion is H, the height from the first surface to the first electrode in contact with the side wall of the ridge portion is 0. A nitride semiconductor laser device, characterized by being 7H or less. The nitride semiconductor laser device according to claim 3,
A height from the first surface to the first electrode in contact with the side wall of the ridge portion is 0.6H or less. The nitride semiconductor laser device according to claim 1,
A nitride semiconductor laser device comprising a second electrode covering the first electrode. The nitride semiconductor laser device according to claim 1,
The nitride semiconductor laser device, wherein a cross-sectional shape of a portion where the side wall of the ridge portion and the first surface are continuous has a skirt shape. The nitride semiconductor laser device according to claim 1,
The nitride semiconductor laser device, wherein the insulating film is formed of any one of an Si oxide film or nitride film, an Al nitride film or oxide film, or a metal oxide film. The nitride semiconductor laser device according to claim 1,
A nitride semiconductor laser element having an adhesive layer with the first electrode on the insulating film. The nitride semiconductor laser device according to claim 8, wherein
The nitride semiconductor laser device, wherein the adhesive layer is formed of an amorphous Si, WO ₃ , Al oxide film or nitride film, or a Ti oxide film or nitride film. A first conductivity type lower cladding layer formed on a substrate;
An active layer formed on the lower cladding layer;
An upper cladding layer of a second conductivity type formed on the active layer and having a conductivity type opposite to the first conductivity type;
A first electrode formed on the upper cladding layer;
The upper clad layer has a ridge shape in which a striped ridge portion projects from the first surface,
Each of the lower cladding layer, the active layer, and the upper cladding layer is a nitride semiconductor laser element made of a nitride semiconductor layer,
The ridge portion has a first portion protruding from the first surface, and a second portion protruding from the first portion and narrower than the width of the first portion,
The first electrode is provided in contact with an upper surface and a side wall of the second portion of the ridge portion, an upper surface of the first portion of the ridge portion, and at least a partial region of the first portion. ,
The nitride semiconductor laser device, wherein the first surface is covered with an insulating film, and the first electrode is not in contact with the first surface. A first conductivity type lower cladding layer formed on a substrate;
An active layer formed on the lower cladding layer;
An upper cladding layer of a second conductivity type formed on the active layer and having a conductivity type opposite to the first conductivity type;
A first electrode formed on the upper cladding layer;
The upper clad layer has a ridge shape in which a striped ridge portion projects from the first surface,
Each of the lower cladding layer, the active layer, and the upper cladding layer is a nitride semiconductor laser element made of a nitride semiconductor layer,
At least two or more ridge portions are provided,
The nitride semiconductor laser device, wherein the first electrode is provided in contact with an upper surface of each ridge portion and at least a partial region of a side wall of each ridge portion. The nitride semiconductor laser device according to claim 11,
The first surface includes a first region sandwiched between the ridge portions and a second region not sandwiched between the ridge portions,
The first electrode is in contact with the first region of the first surface,
2. The nitride semiconductor laser device according to claim 1, wherein the second region of the first surface is covered with an insulating film, and the first region is not in contact with the second region. The nitride semiconductor laser device according to claim 11,
The first surface includes a first region sandwiched between the ridge portions and a second region not sandwiched between the ridge portions,
The first and second regions of the first surface are covered with an insulating film, and the first electrode is not in contact with the first and second regions of the first surface. Nitride semiconductor laser device.

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